The present invention relates generally to mass digital data storage systems, and, more particularly, to systems and methods for automatically allowing the wear associated with storage areas in a non-volatile storage system to be spread out across its storage areas.
The use of non-volatile memory systems such as flash memory storage systems is increasing due to the compact physical size of such memory systems, and the ability for non-volatile memory to be repetitively reprogrammed. The compact physical size of flash memory storage systems facilitates the use of such storage systems in devices which are becoming increasingly prevalent. Devices which use flash memory storage systems include, but are not limited to, digital cameras, digital camcorders, digital music players, handheld personal computers, and global positioning devices. The ability to repetitively reprogram non-volatile memory included in flash memory storage systems enables flash memory storage systems to be used and reused.
Although non-volatile memory or, more specifically, non-volatile memory storage cells within flash memory systems may be repetitively programmed and erased, each cell or physical location may only be erased a certain number of times before the cell wears out. In some systems, a cell may be erased up to approximately ten thousand times before the cell is considered to be unusable. In other systems, a cell may be erased up to approximately one hundred thousand times or even up to a million times before the cell is considered to be worn out. When a cell is worn out, thereby causing a loss of use or a significant degradation of performance to a portion of the overall storage volume of the flash memory system, a user of the flash memory system may be adversely affected, as for example through the loss of stored data or the inability to store data.
The wear on cells, or physical locations, within a flash memory system varies depending upon how often each of the cells is programmed. If a cell or, more generally, a memory element, is programmed once and then effectively never reprogrammed, the wear associated with that cell will generally be relatively low. However, if a cell is repetitively written to and erased, the wear associated with that cell will generally be relatively high. As logical block addresses (LBAs) are used by hosts, e.g., systems which access or use a flash memory system, to access data stored in a flash memory system, if a host repeatedly uses the same LBAs to write and overwrite data, the same physical locations or cells within the flash memory system are repeatedly written to and erased, as will be appreciated by those of skill in the art.
When some cells are effectively worn out while other cells are relatively unworn, the existence of the worn out cells generally compromises the overall performance of the flash memory system. In addition to degradation of performance associated with worn out cells themselves, the overall performance of the flash memory system may be adversely affected when an insufficient number of cells which are not worn out are available to store desired data. Often, a flash memory system may be deemed unusable when a critical number of worn out cells are present in the flash memory system, even when many other cells in the flash memory system are relatively unworn.
In order to increase the likelihood that cells within a flash memory system are worn fairly evenly, wear leveling operations are often performed. Wear leveling operations are generally arranged to allow the cells which are associated with particular LBAs to be changed such that the same LBAs are not always associated with the same cells. By changing the cell associations of LBAs, it is less likely that a particular cell may wear out well before other cells wear out.
One conventional wear leveling process involves swapping physical locations to which two relatively large portions of customer or host LBAs are mapped. That is, the LBAs associated with relatively large sections of storage cells are swapped. Such swapping is initiated through a manual command from a customer, e.g., through the use of a host and, as a result, is not transparent to the customer. Also, swapping operations that involve moving data between two relatively large sections of storage cells are time consuming and, hence, inefficient. Additionally, the performance of the overall flash memory system may be adversely affected by swapping operations of a relatively long duration which consume significant resources, as for example time and processing power, associated with the overall flash memory system. Moving data from a first location typically involves copying the data into another location and erasing the data from the first location.
It is possible to avoid wear leveling by simply allowing cells to wear. Once the cells have effectively worn out, the sectors assigned to the cells may be reassigned by mapping the addresses associated with the sectors to spare areas. As the number of spare areas or cells is limited and valuable, there may not always be spare areas to which sectors associated with unusable cells may be mapped. In addition, effectively remapping sectors only after cells have become unusable generally allows performance of the overall flash memory system to degrade.
Therefore, what is desired is a method and an apparatus for efficiently and transparently performing wear leveling within a flash memory storage system. That is, what is needed is an automated wear leveling process which does not adversely affect the performance of a flash memory storage system while promoting more even wear in physical locations associated with the flash memory storage system.